Geer, [Ref. 1] presents a method for constructing a device capable of maintaining a collector of solar energy in the best position for interception of the sun's rays during the course of the day without relying either on outside energy or on the energy gathered by the collector itself. This method consists of mounting a pair of interconnected containers filled with enough low boiling-point liquid to fill one container on opposite edges of and perpendicular to a collector pivoting on a fixed axis. The containers have shields extending above them on their outer sides in such a way that, when the sun's rays are normal to the collector surface, each container will receive the same amount of radiation. When, owing to the sun's westerly movement, the rays are no longer normal to the collector, the container that is farther from the sun will receive a larger amount of radiation and become warmer than the container that is closer to the sun, which will be shaded by its shield. Vaporization in the warmer container will then transfer liquid to the cooler container, causing the apparatus to tilt until both containers are once again receiving equal amounts of radiation, that is, until the sun's rays are once again normal to the collector surface.
A disadvantage in this embodiment of the method is that each container's view of the sky at any one time is limited to about 90.degree. of arc. After sundown, the collector will remain facing the last position from which the sun caused transfer of liquid, and in the morning the whole apparatus must either be reset manually or, as a consequence of the restricted field of view, wait until the sun's rays are high enough above the shield on the eastern container to strike the western container. When this occurs, the western container will transfer liquid to the eastern one until the collector has tipped back into a position from which it can begin tracking, but in the meantime a substantial portion of morning radiation will have gone uncollected.
A method of overcoming this disadvantage has been disclosed by Baer [Ref. 2]. In this method, interconnected elongated canisters containing the volatile liquid are mounted parallel to the collector surface and, as in the method just described, are shielded with shadow bars so as to allow differential heating and consequent transfer of liquid and tilting of the collector with the sun's westerly movement. In addition, however, they are provided with different rates of heat loss by such means as painting them different colors or partially insulating one but not the other so that, when the sun disappears, one will lose heat more slowly than the other and, being warmer, transfer liquid to it. When the canister with the lower rate of heat loss is mounted on the west side of the collector, the transfer of liquid that occurs after sundown will automatically tilt the collector back to an east-facing position so that it can begin collecting energy upon the sun's reappearance the following morning.
A disadvantage in this method of providing automatic reorientation is that differential heat loss may sometimes be impeded by weather conditions, as, for instance, when cloud cover keeps the temperature of the night sky so close to the temperature of the air surrounding the canisters that they do not achieve a sufficient temperature differential to transfer the quantity of liquid required to tip the collector from a west-facing back to an east-facing position.
Like the differential heat-loss method, that of the present invention provides a canister-powered sun tracking device with a means of resetting itself for interception of morning sunlight, but without alteration of or essential reliance upon the heat-loss rates of the canister surfaces. Instead, the heat-collecting surfaces which control the transfer of liquid between canisters are displaced from the canisters themselves to plates connected to them by tubing and located below and on sides opposite the canisters they serve. These plates transfer heat to their canisters by convection and, because of the larger view of the sky afforded them, can find the reappearing sun from almost any position. Because the sun-tracking ability of the pair of canisters no longer depends upon the sun's striking the surface of the canister farther from it while the canister closer to it is being shaded, it is now possible to insulate the canisters completely and to reply entirely on the plates for the differential heating that is required. It is also possible, however, to leave the canister surfaces exposed and fitted with shadow bars as previously described and to use a single heating surface primarily as a means of resetting the tracking device to an east-facing position when the sun appears in the morning.